(a) Field of the Invention
This invention relates to a tensioning device for imparting a tension to a filament drawn from a reel, as the filament is passed to apparatus which employs the filament. Though not exclusively, the invention finds particular application in the tensioning of relatively fine wire drawn from a reel thereof as the wire is passed to coil-winding apparatus--for instance which winds a transformer winding.
(b) Description of the Prior Art
Various types of tensioning device have been designed, taking into account the material of the particular filament being used and the processing apparatus in which the filament subsequently is employed. This tension is in fact the tensile force which must be applied to the filament in order to draw it to the process. An example of a known tensioning device for fine copper wire comprises a plurality of hard circular beads arranged in two series and through which the wire is passed, the beads of the two series being disposed alternately and the axes of the two series being parallel but displaced from one another. A wire passing through the device thus follows a zig-zag path and the internal and external friction so generated as the wire moves creates the tension. Often with fine enamelled copper wire it is found necessary to apply a pre-tension to the wire, by passing the wire between a pair of friction elements such as felt pads, to ensure that the wire is loaded effectively to follow the zig-zag path.
Such a tensioning device is typically located between but separate from both a reel holder for the wire and the machinery which uses the wire in a subsequent process. The tensioning device is however sometimes integrated with the processing machinery. The relative displacement of the axes of the two series of beads has to be adjusted to give the required tension, but the tension is dependent also upon the characteristics of the wire--such as the gauge and the insulation thereon. Thus, adjustment is necessary whenever a reel of wire is changed for another of a different type, even if the same tension is required. Moreover, the friction between the wires and the beads tends rapidly to wear the beads by forming grooves therein, even if the beads are made of hard materials such as ceramics or glass. This leads to the requirement to adjust the device merely to maintain a required tension, during the use of the device with a particular type of wire. In addition, because insulation coatings have minor thickness variations on any given length of wire the grooves worn in the beads give rise to rapidly changing tensions as the wire is pulled through the device. Moreover, the grooves tend to strip the insulation from the wire.